In a waveform of read data output from FDD, a difference in time between the time T1 from the rise edge of read data RD1 to the rise edge of read data RD2 and the time T2 from the rise edge of read data RD2 to the rise edge of read data RD3, T1-T2, is called "waveform asymmetry" or "asymmetry" that is one of important characteristics in reproduction of data from floppy disks by means of FDD.
A conventional method for simply measuring the asymmetry is to directly measure time using an oscilloscope.
Another conventional method for measuring the asymmetry is disclosed in Japanese Patent Laid-Open No. 94271/1989. According to this conventional method, the time between edges of the read data pulse is measured with a clocking circuit. In this method, a reference clock pulse from a reference pulse generator is counted by means of a clocking circuit to determine the time between the read data pulses.
The above prior art methods have suffered from the following problems.
Specifically, in the measuring method using an oscilloscope, images displayed on the oscilloscope are always swung. Therefore, measuring point cannot be easily determined. This requires a lot of time and skill for the measurement. Further, large measurement errors and personal errors are created. Therefore, the conventional methods lack in reliability and are unsuitable for mass production.
Adoption of an oscilloscope having a time measuring function for mass production enables shortening of the measuring time and the measuring accuracy. This, however, poses a problem of increased cost due to necessity of investment in plant and equipment.
On the other hand, the method for measuring the time from edge to edge in read data pulses by means of a clocking circuit causes a counting error of pulses due to a correlation between two clocking circuits. Further, there is a possibility that a quantization error up to .+-. one period of the reference clock occurs at the time of rising to a unit/omission or rounding-off in clocking with the reference clock. In this case, the lower the frequency of the reference clock, the larger the time per period of the reference clock and the larger the error.